Electrophotograhic photosensitive zinc oxide powder mixture

ABSTRACT

An electrophotographic member comprising a support layer and an electrophotographic photosensitive layer disposed on the support layer, the electrophotographic photosensitive layer comprising a power mixture dispersed in an electrically insulative film forming binder material, the powder mixture being a mixture of larger zinc oxide particles and smaller zinc oxide particles, the ratio by weight of the larger particles to the smaller particles being within the range from 4 : 6 to 1 : 9, the larger particles having an average particle diameter of not less than 1.2 microns and the smaller particles having an average particle diameter of not more than 0.8 microns, at least 60 percent by weight of the larger particles having a diameter not less than 1 micron and at least 70 percent by weight of the smaller particles having a diameter not more than 1 micron whereby the particle size distribution curve is characterized by the presence of two peaks respectively corresponding to the smaller and larger particles.

[ Reissued Nov. 25, 1975 ELECTROPHOTOGRAHIC PHOTOSENSITIVE ZINC OXIDEPOWDER MIXTURE [75] Inventor:

[73] Assignee: Fuji Photo Film Co., Ltd.,

Minami-ashigara, Japan [22] Filed: Jan. 13, 1975 [21] Appl. No.: 540,687I Related U.S. Patent Documents Hajime Miyatsuka, Asaki, Japan [51] Int.Cl.

v G03G 5/087 [58] Field of Search...

[56] References Cited UNITED STATES PATENTS 3,121,007 2/1964 Middletonet a1. 96/l.8 X 3,151,982 /1964 Corrsin 3,198,632 8/1965 Kimble et al.96/l.8 X

PARTlCLE SIZE DlSTRlBUTlON We) 5 3,467,497 9/1969 Weisbach et al. 96/1.8X

Primary Examiner-Norman G. Torchin Assistant ExaminerJohn R. MillerAttorney, Agent, or FirmGerald J. Ferguson, Jr.;

Joseph J. Baker [5 7] ABSTRACT An electrophotographic member comprisinga support layer and an electrophotographic photosensitive layer disposedon the support layer, the electrophotographic photosensitive layercomprising a power mixture dispersed in an electrically insulative filmforming binder material, the powder mixture being a mixture of largerzinc oxide particles and smaller zinc oxide particles, the ratio byweight of the larger particles to the smaller particles being within therange from 4: 6 to 1 9, the larger particles having an average particlediameter of not less than 1.2 microns and the smaller particles havingan average particle diameter of not more than 0.8 microns, at leastpercent by weight of the larger particles having a diameter not lessthan 1 micron and at least percent by weight of the smaller particleshaving a diameter not more than 1 micron whereby the particle sizedistribution curve is characterized by the presence of two peaksrespectively corresponding to the smaller and larger particles.

4 Claims, 2 Drawing Figures PARTlCLE S|ZE 1p) Reissued Nov.25, 1975 Re.28,626

FIG. I

PARTlCLE SlZE D\STR\BUT\ON We) PARTICLE S\ZE (p) FIG. 2

PARTKJLE S\ZE D\STR\BUT\ON We) PARTlCLE SIZE (p) INVENTOR HAJIMEMIYATSUKA BY FERGC/JO/Vg E/IKE/Z ELECTROPHOTOGRAHIC PHOTOSENSITIVE ZINCOXIDE POWDER MIXTURE Matter enclosed in heavy brackets I 1 appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1 and 2 are graphs showing theparticle size distribution of photoconductive zinc oxide powder employedin the electrophotographic photosensitive material according to thisinvention.

DETAILED DESCRIPTION OF INVENTION This invention relates to anelectrophotographic photosensitive material and particularly tophotoconductive zinc oxide employed as a component in a compositionforming electrophotographic photosensitive layer.

In the ordinary electrophotographic process consisting of electrostaticcharging, imagewise exposure and toner development, thetone-reproduction characteristics are considerably deteriorated by thewell-known so-called edge effects, a phenomenon in which theelectrostatic image having a wide solid area is developed with the imagedensity being higher in the peripheral portion thereof than in thecentral portion thereof, resulting in the reproduced image having'exaggeratedly intensified contours.

The edge effect is most usually prevented by the use of a developmentelectrode. Though the use of a development electrode is quite effectivefor removing said edge effects, the effectiveness of said electrode isconsiderably dependent upon the distance between the photosensitivelayer and said electrode. It is desirable to maintain said distance assmall as possible, but realization of such a small distance as to becapable of giving satisfactory preventive effect involves considerabletechnical difficulties. Moreover, the dependence on the distance betweenthe photosensitive layer and'the development electrode becomes moremarked when said photosensitive layer is provided with a smooth surfaceand in this case, therefore, it becomes necessary to maintain quiteprecisely said distance between the photosensitive layer and developmentelectrode at an extremely small value in order to obtain a reproducedimage of high quality without edge effects. It will be readilyunderstood that this will be technically difficult.

Photoconductive zinc oxide powder employed in electrophotographygenerally shows, when prepared into a photosensitive layer, higher lightsensitivity as the diameter of particles in-creases. Zinc oxide ofexcessive large diameter will, however, lead to porous dispersionthereof on the photosensitive layer, giving a rise to an extremelycoarse surface in which the surface of support material is exposedlocally. Such a surface is characterized by an elevated leaking speed ofelectrostatic charge disposed thereon, leading to an unsatisfactorilylow density image. At the same time exact reproduction of the originalpattern becomes impossible because of the elevated coarseness of thesurface, and thereproduction of halftone or continuous tone imagesbecomes extremely difficult even if the edge effects can be successfullyprevented. From these reasons the practical particle diameter ofphotoconductive zinc oxide employed in electrophotography is limited toa certain value, though the light sensitivity is higher as the diameterincreases. Said limit is also dependent on the thickness of theelectrophotographic photosensitive layer, and usually does not exceedca. 1.0 micron for a layer thickness of ca. 10 microns employed inordinary electrofax paper.

In electrophotography toner streaking and fogging are the major factorswhich deteriorate the quality of the developed image. The phenomena ofsuch toner streaking and fogging though naturally dependent on theproperties of developer itself, are greatly influenced by thecharacteristics of photosensitive layer. For example, fogging due toretentive potential becomes a problem when the content of binder resinpresent in the photosensitive layer is large. Also the characteristicsof the zinc oxide powder itself are sometimes related directly with theformation of retentive potential, and the composition of thephotosensitive layer may also become the cause of said fogging or tonerstreaking. It is also empirically known that the phenomena of tonerstreaking and fogging are considerably influenced by, besides thecomposition of photosensitive layer itself, the shape of the surface ofthe photosensitive layer, and are apt to appear more frequently in thephotosensitive layer provided with a smooth surface.

The object of this invention is to provide an electrophotographicphotosensitive material capable of being developed with liquid developerand capable of effectively removing said edge effects without muchdependence on the distance between the development electrode and saidthe photosensitive layer.

Another object of this invention is to provide an electrophotographicphotosensitive material with high light sensitivity containingphotoconductive zinc oxide powders of large particle size which cannotbe employed alone due to poor dispersibility thereof in thephotosensitive layer.

Further another object of this invention is to provide anelectrophotographic photosensitive material relatively free from thephenomena of toner streaking and fogging. V

The above-mentioned objects of this invention are achieved by providingan electrophotographic photosensitive layer essentially composed offinely divided, photoconductive zinc oxide powder and an insulative filmforming material provided on a support material which comprises the useof a mixture of larger zinc oxide particle powders containing particlesof a diameter not less than 1.0 micron at a content at least of 60 wtpercent and having an average particle diameter not less than 1.2microns and of smaller zinc oxide particle powders containing particlesof diameter not exceeding 1.0 micron at a content at least of wt percentand having an average particle diameter not more than 0.8 microns assaid photoconductive zinc oxide powder. The mixing ratio of said largerparticle zinc oxide powder and said smaller zinc oxide powder is withina range from 4 6 to l 9. Thus the electrophotographic photo-" sensitivematerial according to this invention can be prepared by applying onto asupport material consisting of a paper sheet or resin film provided withan electroconductive layer thereon. According to the invention asdescribed above, a solution is obtained by dispersing the mixture ofphotoconductive zinc oxide powder homogeneously with a resin componentas the insulative film forming material by means of dispersingequipment.

In the case of electrophotographic photosensitive layer employingexclusively larger particle zinc oxide powder having average particlediameter of not less than 1.2 microns as said photoconductive zinc oxidepowder, the dispersibility of Zinc oxide in the photosensitive layer isquite unsatisfactory at a layer thickness of ca. microns. On the otherhand the disperson of zinc oxide in the photosensitive layer can beimproved by mixing larger particle zinc oxide powder with smallerparticle zinc oxide powder according to this invention so as to fill thecavities between the larger particles with smaller particles thereby toform a photosensitive layer of a continuous phase.

In order to obtain such an improved dispersion state the particles sizesof larger particle zinc oxide powder and smaller particle zinc oxidepowder are subject to certain limitation, and such improved dispersioncan be obtained effectively by choosing zinc oxide powder havingparticle sizes located within said limitations. According to theexperiments carried out by the present inventors, it is confirmed thatthe average particle diameter of larger particle zinc oxide powder andof smaller particle zinc oxide powder should be not less than 1.2microns and not more than 0.8 microns respectively, when measured bypermeability method.

The advantages of this invention lie in the facts that larger particlezinc oxide having average particle diameter not less than 1.2 microns,which cannot be employed alone as photoconductive zinc oxide forelectrophotography due to the drawbacks mentioned above, is madeutilizable for this purpose by means of mixing with smaller particlezinc oxide powder thereby not only utilizing the elevated specificsensitivity thereof but also obtaining other advantages as described inthe following.

The photosensitive layer of this invention is characterized by thesurface shape thereof. Namely, in comparison with the surface ofordinary photosensitive.

layer containing sufficiently dispersed zinc oxide therein, the surfaceof the photosensitive layer obtained according to this invention isprovided with considerably higher coarseness to give matted appearance.Furthermore a close examination for example by means of opticalmicroscope with readily reveal the singularity of the surface shape ofthe photosensitive layer according to this invention. In the surface ofthe photosensitive layer of ordinary electrofax paper, the minimum unitof coarseness thereon is so small as to be almost unmeasurable byoptical microscope, thereby giving a very smooth surface with minimumcoarseness. On the contrary in the photosensitive layer obtainedaccording to this invention, the surface coarseness shows measurably alarge minimum unit and is essentially determined by spherical blocks ofseveral tens of microns in diameter which are presumed to be composed ofagglomerated zinc oxide particles. Furthermore in said photosensitivelayer said spherical agglomerated blocks are distributed uniformly andcontinuously to show the surface of unique "appearance that iscompletely different from that of ordinary electrofax paper.

The photosensitive layer of this invention has an appearance ofhomogeneous continuous phase under observation with bare eyes and thedeveloped image is not deteriorated by the coarse surface despite therelatively large unit of coarseness thereof, because said agglomeratedblocks constitute the minimum of coarseness and presumably are composedessentially of zinc oxide 4 and are. uniformly distributed on thephotosensitive layer to form a surface which may be referred to asuniform coarse surface. Matted appearance is also another unique pointof the photosensitive layer provided with. a uniform coarse surfaceaccording to this invention.

The uniform coarse surface" of the photosensitive layer according tothis invention results in various advantages as follows.

In the first place the photosensitive layer of this invention can havehigher whiteness by the light scattering effect due to the largercoarseness of the surface thereof. Besides the photosensitive layer ofthis invention is capable of effectively preventing the aforementionededge effect irrespective of the distance between the developmentelectrode and the photosensitive layer.

Furthermore, the photosensitive layer of this invention is provided withan advantage of reduced toner streaking and lower fogging level comparedwith ordinary smooth photosensitive layer. Therefore, the photosensitivelayer of this invention can be used with several kinds of liquiddevelopers.

Moreover, the photosensitive layer of this invention is 'provided withanother important advantage, compared with the other photosensitivelayer with a smooth surface such as in ordinary electrofax paper, thatis, the toner image formed thereon is barely affected by the frictionwith another plane. This is attributable to the fact that the uniformcoarse surface has much fewer contact points with the other planecompared with a so-called smooth surface. The toner image on thephotosensitive layer shows considerably low mechanical strengthparticularly in a liquid developer due to swelling, etc., and easilyforms scratches by the contact with developing electrode, etc., thusalso rendering it impossible to shorten the distance to the developingelectrode beyond a certain limit. In the photosensitive layer of thisinvention, however, the toner image formed thereon is so strong as toenable developing even under direct contact with developing electrode.Consequently the tone-reproduction of the photosensitive layer in thisinvention can be remarkably improved by minimizing the distance betweenthe photosensitive layer and development electrode, namely by contactingthe developing electrode with the photosensitive layer, and this factalso constitutes another important advantage of the photosensitive layeraccording to this invention.

As already explained above, the various advantages of the photosensitivelayer of this invention are derived from the fact that saidphotosensitive layer is provided with uniform coarse surface. Thephotosensitive layer provided with a uniform coarse surface" can be mosteffectively be achieved by the process of this in vention, and can neverbe prepared by single zinc oxide powder. For example, in case ofemploying a zinc oxide having a small average particle diameter andprovided with a surface sufficiently wettable by organic solvents, awell dispersing coating solution only gives a photosensitive layerprovided with an extrememly smooth and uniform surface, whereas thecoating solution prepared by extremely little dispersion provides asmooth surface with layer blocks scattered thereon which arecharacterized by relatively large agglomerates (diameter ca. microns)and which gives rise to the formation of visually distinguishable spotsupon development. Such a surface which does not allow visuallyunithereby deteriorating remarkably the quality of said image.

The photoconductive zinc oxide powder employed in this inventionconsists of a mixture of two powders of larger particle size and smallerparticle size, respectively, and thus the particle size distributioncurve photoconductive zinc oxide powder to be employed in this inventionis characterized by two peaks located at the larger and smaller particlesize respectively and having heights corresponding to the mixing ratiothereof. Such unique size distribution pattern cannot be obtained in asingle zinc oxide powder. For example, zinc oxide powder produced by thephase oxidation method and utilized commonly in the electrophotographicprocess usually has a particle size distribution curve provided withsingle peak.

The presence of two peaks in the particle size distribution curve seemsto render the zinc oxide powder to be employed in this inventionparticularly suitable for obtaining a uniform coarse surface. Forexample, in the process of this invention, when the amount of eitherzinc oxide powder, either larger sized one or smaller sized one, isdecreased thereby lowering the corresponding peak in the distributionpattern to reach a powder substantially consisting of zinc oxide powderof a single species, the photosensitive layer obtained shows eitheruneven coarse surface" or smooth surface and becomes not different fromthat obtained from zinc oxide powder of single species. From such apoint of view the mixing ratio of larger particle and smaller particlezinc oxide powder is considered important in order to obtain thephotosensitive layer provided with uniform coarse surface which is theprincipal characteristic of said layer according to this invention, andthe optimum mixing ratio by weight of said larger sized powder andsmaller sized one is found to be located within a range of 4 6 to l 9,according to the experiments of the present inventors.

In the mixture of photoconductive zinc oxide powder employed in thisinvention, the zinc oxide powder of larger diameter is preferred to havean average particle diameter not less than 1 .2 microns whereas that ofsmaller diameter is preferred to have an average particle diameter notmore than 0.8 microns. Furthermore, in order to obtain a photosensitivelayer provided with uniform coarse surface by a mixture of powder oflarger particles and smaller particles, it is found to be necessary thatsaid powder of larger particles contains particles with diameter notless than 1 micron at a content not less than 60 wt percent of saidpowder and that said powder of smaller particles contains particles withdiameter not more than 1 micron at a content not less than 70 wt percentof said powder. This fact was confirmed as the results of experimentsusing various mixtures of zinc oxide powder.

In fact the photoconductive zinc oxide powder employed inelectrophotography is almost exclusively produced by gas phase oxidationprocess and therefore has considerably narrow particle size distributionrange represented by sharp size distribution curve. Conse- 6 quently thecondition mentioned above is almost invariablyIsatisfied if the averageparticle diameter is well defined.

Conclusively the aforementioned advantages of the photosensitive layeraccording to this invention are derived from a uniform coarse surface,which can be effectively realized by the fact that the zinc oxide powderemployed consists of a mixture of two species of powders provided withlarger particle size and smaller particle size respectively, having twopeaks in the size distribution corve. Inversely stated, a simple andeffective method for producing a uniform coarse surface is provided bythis invention.

The morphological characteristics of the photosensitive layer providedwith uniform coarse surface according to this invention are furtherclarified by the observation by optical microscope.

Namely in this case, the surface consists of agglomerates havingdiameter of 10 to 50 microns and presumed to be essentially composed ofaggregated zinc oxide powder, with the spacing less than microns betweensaid agglomerates.

The surface of such condition is defined as the uniform coarse surfacewhich is effective for realizing the objects of this invention and alsorealizing various advantages of the photosensitive layer according tothis invention.

This invention is further provided with an advantage in that themanufacture of a photosensitive layer can be carried out under ordinaryconditions without any limitation over the selection of binder resin,dispersing method etc.

It is to be noted, however, that the composition of the photosensitivelayer should be sufficiently homogeneous over the whole surface thereof.For example an excessively mild dispersing condition leads to theformation of giant particles (ca. 100 microns) scattered in the regularbackground consisting of agglomerates of diameter 30-50 microns, andsuch formation should be prevented because said giant particles giverise to the formation of visually perceivable spots.

Thus it is important that the frequency of protruding portions withparticle diameter exceeding 100 microns should not exceed 5/m preferably1 or 2/m when examined over a wide surface under an optical microscope.

Also it is confirmed, by our experience, that the zinc oxidephotosensitive layer is provided with an ideal surface structure such asmentioned above by the process of this invention more easily than byvarious other methods.

The result of the process of this invention is further assured byfiltering the coating solution before application to the supportmaterial with filter cloth or metal filter of more than 200 mesh therebyremoving large blocks contained therein. Said filtration preventseventual contamination of photosensitive layer with large block of ca.100 microns and assures the formation of a photosensitive layer providedwith a uniform coarse surface.

In order to obtain a photosensitive layer capable of being developedwith a liquid developer and of providing the reproduction of continuoustone images, it is important to choose a binder resin capable ofretaining electrostatic charge for a prolonged period in various liquiddevelopers.

Preferred binder is determined in consideration of the charge retentiveability thereof in various insulative 7 iquids, but most thermoplasticresins can be satisfactoily utilized for this purpose when the carrierliquid :onsists of isoparaffinic hydro-carbon with relatively owdissolving power. As an example of such thermo- Jlastic resins arepolyacrylic esters, polymethac'ryli'c asters, copolymers of accrylicesters with styrene, vinyl acetate or methacrylic esters, or vinylchloride-vinyl acetate polymer. On. the other hand when the carrieriquid is provided with higher dissolving power such as :yclohexane,kerosene, decaline or aliphatic hydrocaraon .containing a small amountof aromatic hydrocaraon etc., resins abundantly containing copolymercom- )onents having an elevated affinity to non-polar solrents such asstyrene or butyl methacryelate show in- ;ufficient charge retentiveability, and therefore pre- :erred are the resins with crosslikedstructure such as alkyd resin, epoxyester resins, epoxy resin, polyure-:hane resins, particularly alkyd resins or epoxyester resns crosslinkedwith polyisocyanate.

The mixing ratio by weight of photoconductive material and binder ispreferred to be within a range of 1:1 10 :1, and most practicallyemployed is a ratio be- :ween 3:1 and 10:1.

The present invention will be further clarified by the Followingexamples.

In the following examples prepared under identical :onditions werephotosensitive layers employing single species of zinc oxide of eitherlarger particle size or smaller particle size exclusively.

The photosensitive layer prepared with zinc oxide powder of smallerparticle size (exclusively employing zinc oxide having average particlesize of 0.62 microns as photoconductive powdered material) is referredto as photosensitive layer A.

The photosensitive layer prepared with zinc oxide powder of largerparticle size (exclusively employing zinc oxide having average particlesize of 1.98 microns as photoconductive powdered material) is referredto as photosensitive layer B.

Furthermore the photosensitive layer prepared with a mixture consistingof 700 parts by weight of said ;maller particle zinc oxide powder and300 parts by weight of said larger particle zinc oxide powder (employing the mixture of zinc oxide powders having average particle sizesof 0.62 and 1.98 microns respectively as photoconductive powderedmaterial) is referred to as photosensitive layer C.

On the thus prepared photosensitive layers A, B and 3 compared were thecharging properties including the .nitial potential (V dark decaypotential retentive rate (V /V and developing properties including :onerstreaking and fogging density, of which results are shown in Table 1.

At the measurements of various properties each pho- :osensitive layerwas conditioned in a dark place for nore than 2 full days prior to themeasurements.

EXAMPLE 1 1,000 parts by weight of photoconductive zinc oxide nixedpowder consisting of 700 parts by weight of ;maller particle zinc oxidepowder (average particle diparticle zinc oxide powder (average particlediameter 1.98 microns) is'added with 240 parts by weight of styrenizedalkyd resin varnish (Japan Reichhold; Styre- S61 4400), parts by weightof polyisocyanate varnish (bayer AG; Desmodul L) and butyl acetate anddispersed sufficiently for ca. 30 minutes in a homogenizer. The coatingsolution thus prepared was then applied on a polyethylene terephthalatefilm provided with evaporated aluminum layer thereon (Toray Co.;Metalmy) so as to obtain a coating of 5 microns thick after drying.Drying was carried out at 50 C for more than 16 hours so as to realizehardening simultaneously with drying.

The photosensitive layer thus prepared is provided with the surface ofmatted apprarance, on which semispherical agglomerated blocks ofdiameter of 30 50 microns are distributed continuously and uniformly toform uniform coarse surface. The zinc oxide mixed powder employed inthis example has a peculiar parti cle size distribution of FIG. 1showing two maximum peaks at larger and smaller particle siderespectively. The particle size distribution of each powder is alsoshown in FIG. 1 by dotted lines a and b respectively for smaller sizeand larger size zinc oxide powder.

The measurement of charging characteristics was carried out as follows.The sample was charged negatively with negative corona discharge, thensubjected to the measurement of initial potential V in air, successivelyimmersed in kerosene and again subjected to the measurement of retentivepotential V 60 seconds after the measurement of V The dark decaypotential retentive rate is defined by V devided by V The developeremployed consisted of 4 g commercially available offset ink (Toyo lnkManufacturing Co., Ltd.; blue ink) dispersed in l l. of kerosene, andthe developing characteristics were evaluated on the developed lineimage with respect to the toner streaking and fogging density afterdeveloping for ca. 2 minutes in a stainless steel tray.

The photosensitive layer B is provided with extremely coarse surfacewith many cavities and lost the electrostatic potential thereof almostcompletely in the liquid developer thereby giving very low developingdensity and rendering the observation of toner streaking and foggingdensity meaningless.

This comparison in Table 1 shows that the photosensitive layer C issuperior with respect to the other streaking and fogging density.

EXAMPLE 2 1,000 parts by weight of photoconductive mixed powder materialconsisting of 650 parts by weight of smaller size zinc oxide (averageparticle diameter 0.4 microns) and 350 parts by weight of larger sizezinc oxide (average particle diameter 1.98 microns) is added with 240parts by weight of styrenized alkyd resin varnish (Japan Reichhold Co.;Styresol 4,400),

110 parts by weight of polyisocyanate varnish (Bayer AG, Desmodul L) anda solution containing 0.2 parts by weight of Brilliant Blue FCF,

small particle 0.4 parts by weight of eosin and 0.5 parts by weight offluorescein as sensitizing dyes dissolved in 40 parts by weight ofmethanol.

The mixture is further added with butyl acetate and sufficientlydispersed for ca. 30 minutes in a homogenizer. The coating solution thusprepared was applied on Metalmy similarly as in Example 1, then driedand hardened to obtain electrophotographic photosensitive layer. Thephotosensitive layer thus prepared is provided with uniform coarsesurface and has sensitivity in the visible wavelength range. Theparticle size distribution of the mixed zinc oxide powder is representedby the full line in FIG. 2, which also shows the size distribution ofsmaller and larger size zinc oxide powder by the dotted lines a and b,respectively.

What is claimed is:

1. An electrophotographic member comprising a support layer and anelectrophotographic photosensitive layer disposed on said support layer,said electrophotographic photosensitive layer comprising a powdermixture dispersed in an electrically insulative film forming bindermaterial, said powder mixture being a mixture of larger zinc oxideparticles and smaller zinc TABLE 1 Dark decay potential lnitialretentive hotosensiti e potential rate Toner Photosensitive layer ZnOpowder (V,,) (Vim/V streaking Fogging density surface Layer A Smallparticle (average 0.62p.) l80 0.90 Considerable High Smooth Layer BLarge particle (average l.98/1.) l 0.23 Extremely coarse Layer C Mixtureof large particle and l50 0.85 Scarce Extremely low Uniformly coarseoxide particles, the ratio by weight of said larger particles to saidsmaller particles being within the range from 4 6 to l 9, said largerparticles having an average particle diameter of not less than 1.2microns and said smaller particles having an average particle diameterof not more than 0.8 microns, at least 60 percent by weight of saidlarger particles having a diameter not less than 1 micron and at least70 percent by weight of said smaller particles having a diameter notmore than 1 micron whereby the particle size distribution curve ischaracterized by the presence of two peaks respectively corresponding tosaid smaller and larger particles. 2. An electrophotographicphotoconductive layer as claimed-in claim 1, wherein saidphotoconductive zinc oxide is formed by gas phase oxidation process.

material is epoxyester resin cured by polyisocyanate.

1. AN ELECTROPHOTOGRAPHIC MEMBER COMPRISING A SUPPORT LAYER AND ANELECTROPHOTOGRAPHIC PHOTOSENSITIVE LAYER DISPOSED ON SAID SUPPORT LAYER,SAID ELECTROPHOTOGRAPHIC PHOTOSENSITIVE LAYER COMPRISING A POWDERMIXTURE DISPERSED IN AN ELCTRICALLY INSULATIVE FILM FORMING BINDERMATERIAL, SAID POWDER MIXTURE BEING A MIXTURE OF LARGER ZINC OXIDEPARTICLES AND SMALLER ZINC OXIDE PARTICLES, THE RATIO BY WEIGHT OF SAIDLARGER PARTICLES TO SAID SMALLER PARTICLES BEING WITHIN THE RANGE FROM4:6 TO 1:9 SAID LARGER PARTICLES HAVING AN AVERAGE PARTICLE DIAMETER OFNOT LESS THAN 1.2 MICRONS AND SAID SMALLER PARTICLES HAVING AN AVERAGEPARTICLE DIAMETER OF NOT MORE THAN 0.8 MICRONS, AT LEAST 60 PERCENT BYWEIGHT OF SAID LARGER PARTICLES HAVING A DIAMETER NOT LESS THAN 1 MICRONAND AT LEAST 70 PERCENT BY WEIGHT OF SAID SMALLER PARTICLES HAVING ADIAMETER NOT MORE THAN 1 MICRON. WHEREBY THE PARTICLE SIZE DISTRIBUTIONCURVE IS CHARACTERIZED BY THE PRESENCE OF TWO PEAKS RESPECTIVELYCORRESPONDING TO SAID SMALLER AND LARGER PARTICLES.
 2. Anelectrophotographic photoconductive layer as claimed in claim 1, whereinsaid photoconductive zinc oxide is formed by gas phase oxidationprocess.
 3. An electrophotographic photoconductive layer as claimed inclaim 1, wherein said insulating film forming material is alkyd resincured by polyisocyanate.
 4. An electrophotographic photoconductive layeras claimed in claim 1, wherein said insulating film forming material isepoxyester resin cured by polyisocyanate.